U.S. Pat. No. 5,634,169 to Barry et al. discloses a coupling of two rotatable members through a torsion spring. The first rotatable member is drive member of predetermined torque and the second, driven member drives a paddle in a toner hopper. The amount of deflection of the torsion spring is employed to sense the amount of toner in a hopper. In such an application, the torque characteristics of the spring must be within predetermined limits or the measurement of toner will not be sufficiently accurate.
Accordingly, characteristics of springs for such a use are measured so as to grade the spring as within acceptable range of characteristics or not. The previous known methods of grading torsion springs measures spring characteristics at particular amounts of deflection. However, this has proved not to be an accurate method of measurement.